1. Field of the Invention
The present invention relates to an electrophotographic photoconductor for use in the copying machines and printers employing the electrophotographic process, and more particularly to an electrophotographic photoconductor comprising a photoconductive layer which contains a specific phenol compound.
2. Discussion of Background
Inorganic compounds such as selenium, selenium tellurium alloy and arsenic selenium are conventionally used as photoconductive materials for use in the electrophotographic photoconductor. However, such inorganic photoconductive materials have toxicity, thereby causing the environmental problems. In addition, the properties of the above-mentioned inorganic photoconductive materials tend to readily deteriorate. This is because, for instance, selenium, which is in an amorphous condition when used as the photoconductive material, easily tends to crystallize when inappropriately heated or stained with dirt. Further, such inorganic photoconductive materials must be formed into a thin film with a thickness of several tens of micrometers by vacuum deposition, thereby increasing the manufacturing cost of the photoconductor.
To solve the above-mentioned problems of the inorganic photoconductive materials, electrophotographic photoconductors using the organic photoconductive materials have been actively developed in recent years, and many organic photoconductors have been put into practice.
The electrophotographic photoconductors employing the organic photoconductive materials have the advantages that the manufacturing cost is cheap and the mass-production is possible. However, the photoconductive layer comprising such organic photoconductive materials tends to easily deteriorate during the repeated electrophotographic process. To be more specific, the electrophotographic process includes the steps of charging the surface of the photoconductor by corona charging and exposing the photoconductor to light images to form latent electrostatic images thereon. Then, the latent electrostatic images formed on the surface of the photoconductor are developed with a toner, and the developed toner images are transferred to a sheet of paper. In addition, the residual toner remaining on the photoconductor after the transfer step is removed therefrom by the cleaning step. As mentioned above, various external forces are chemically and mechanically applied to the surface of the photoconductor. As a result, the electrostatic properties of the photoconductive layer are caused to deteriorate, and the surface of the photoconductor is easily worn out and scratched. Therefore, the organic photoconductor is required to have sufficient durability to cope with the above-mentioned applied chemical and mechanical external forces.
To protect the surface of the photoconductor from the mechanical force, the method of adding a lubricant to a surface layer and a protective layer of the photoconductor to decrease the coefficient of friction of the surface portion thereof is considered to be effective, as disclosed in Japanese Laid-Open Patent Applications 52-117134 and 63-61256.
Further, when the electrophotographic photoconductor is repeatedly subjected to the electrophotographic process, the charging potential and the sensitivity are decreased and the residual potential is increased. The above-mentioned problems are caused by the influence of gases such as ozone and nitric oxide (NO.sub.x) generated by the corona charging in the course of the charging step.
To solve the above-mentioned problems, it is proposed to add a particular antioxidant to the photoconductive layer and the protective layer provided on the photoconductive layer, for example, as disclosed in Japanese Laid-Open Patent Applications 4-184455, 6-175381 and 3-188456. However, satisfactory results cannot be obtained by the addition of such conventional antioxidants.
In recent years, the copying machine is required to produce high quality images and be provided with text editing function and composite processing function. In line with the above-mentioned demands, non-impact printing technology has been developed and digital recording apparatuses such as a laser printer, laser facsimile machine and digital copying machine have been widely utilized.
Most of the above-mentioned digital recording apparatuses employ as a light source semiconductor laser because it is compact, cheap and convenient. The wavelength of the currently used semiconductor laser beam is limited to the range of the near infrared rays of 750 nm or more, so that the electrophotographic photoconductors used in the above-mentioned digital recording apparatuses are required to show sufficient photosensitivity in the wavelength range of at least 750 to 850 nm.
It is conventionally known that the organic photoconductive materials, for example, a phthalocyanine pigment, azo pigment, cyanine pigment, azulene pigment, and squarylium pigment can show the photosensitivity in the above-mentioned wavelength range. In particular, the phthalocyanine pigment can show absorption and photosensitivity in the relatively long wavelength range. In addition, a variety of phthalocyanine pigments can be obtained according to the kind of central metal or the type of crystalline form. Therefore, research and development of the phthalocyanine pigment has been actively conducted to cope with the semiconductor laser for use in the electrophotographic photoconductor.
There are conventionally known .epsilon.-type copper phthalocyanine, X-type metal-free phthalocyanine, .tau.-type metal-free phthalocyanine, vanadyl phthalocyanine and oxytitanyl phthalocyanine. Any of the above-mentioned phthalocyanine compounds are still insufficient in terms of photosensitivity, chargeability, and the durability in the repeated use.
To eliminate the shortcomings of the above-mentioned conventional phthalocyanine compounds, oxytitanyl phthalocyanine pigments provided with high photosensitivity are proposed, as disclosed in Japanese Laid-Open Patent Application 59-49544 (U.S. Pat. No. 4,444,861), Japanese Laid-Open Patent Application 59-166959, Japanese Laid-Open Patent Application 61-239248 (U.S. Pat. No. 4,728,592), Japanese Laid-Open Patent Application 62-67094 (U.S. Pat. No. 4,664,997), Japanese Laid-Open Patent Application 62-275272, Japanese Laid-Open Patent Application 63-366, Japanese Laid-Open Patent Application 63-116158, Japanese Laid-Open Patent Application 63-198067, Japanese Laid-Open Patent Application 64-17066, Japanese Laid-Open Patent Application 2-28265, Japanese Laid-Open Patent Application 3-35064, and Japanese Laid-Open Patent Application 3-200790.
In the electrophotographic photoconductor, all the charge transport materials do not always show good performance when used in combination with a specific charge generation material. Namely, there is a preferable combination of a charge transport material and an additive such as an antioxidant with the specific charge generation material. If those materials are improperly used in combination, there occur many problems, for example, decrease of the photosensitivity, increase of the residual potential, and deterioration of the charging stability.
According to Japanese Laid-Open Patent Applications 1-82043, 2-136862 and 2-189555, preferable combinations of the above-mentioned oxytitanyl phthalocyanine pigment with the specific charge transport materials are proposed. However, any electrophotographic photoconductors are still unsatisfactory in terms of the photosensitivity, residual potential and the charging stability in the repeated use.